1) TECHNICAL FIELD
The present invention relates to a process for extracting and concentrating elemental rhodium from both primary and secondary sources.
2) BACKGROUND INFORMATION
Heretofore, in order to extract, concentrate, and purify rhodium, several different methods have been employed. Primary sources of rhodium include its ores or any other material which is removed from the Earth and contains rhodium in any one of its native forms. Secondary sources of rhodium include all materials from which rhodium may be refined which have been previously employed for another purpose, e.g. spent catalyst materials, platinum/rhodium thermocouple alloys, electrical contact points, etc. For the recovery of rhodium from primary sources, ores are generally crushed, finely ground and then treated by flotation and magnetic methods to separate sulphide minerals. These sulphides are further separated to yield a nickel concentrate which contains most of the platinum metals. Selective removal of copper followed by controlled oxidation of sulphur leaves behind nickel which contains platinum metals as impurities. This nickel is refined electrolytically, and the platinum metals are recovered from the anode slimes. In the processing of the anode slimes a method is required whereby the platinum metals are converted into solutions of their ions. Once in the form of ionic aqueous solutions, the platinum metals are separated and purified by means known to those skilled in the art.
In the case of secondary sources which contain rhodium, several different methods are available to the refiner for its recovery. The most popular of these methods are described below. The method of recovery chosen depends upon the type of secondary material from which rhodium is being recovered. For example, in the case of certain platinum/rhodium alloy thermocouple wires, it is practical to dissolve the alloys in aqua regia. However, the use of aqua regia becomes less practical when small amounts of rhodium are present in a large amount of insoluble material such as the ceramic support material in the case of automotive catalyst materials.
Since most ores and secondary sources of rhodium contain rhodium as a minor constituent of a mixture, and since it is costly to extract rhodium on a large scale from most mixtures in which it is only present as a minor component, it is of advantage to have at hand a useful method whereby rhodium may be extracted from its sources and rendered into a more concentrated form as an intermediate step prior to final processing. A major burden to this end in the past has been the exceptional difficulty of rendering rhodium soluble to aqueous solution.
One method which is widely used to render rhodium soluble comprises exposing rhodium to sulfuric acid at or near the boiling point of sulfuric acid with or without the aid of other reagents such as sulfur trioxide gas. Another method comprises heating rhodium and a chloride salt of an alkalai or an alkaline Earth metal in an atmosphere of chlorine gas to a high temperature. A third published method comprises the mixing of rhodium with sodium bisulfate and heating to the fusion temperature of the latter until the rhodium is oxidized. Another method has been described by Hirose in U.S. Pat. No. 4,859,445 whereby rhodium is exposed to a solution of hydrochloric acid while highly toxic chlorine gas is bubbled through the solution. Finally, it is known that hot hydrochloric acid dissolves rhodium, but at such a slow rate to be of little practical use.
The present invention provides a method whereby rhodium is dissolved from its sources and concentrated and at the cathode of an electrolytic cell without the need for extreme temperatures, on-site storage of highly hazardous materials, or highly specialized apparatus.